...
Sync Newsletter Subscribers
If you want Maileon to send you the DOI letter, you have to enable the DOI process both in Shopify and in the App settings.
If you want Shopify to send the DOI letter, enable it in Shopify but not in the App settings.
If you do not want DOI process, then disable it in both Shopify and Maileon
Sync Order Confirmations
Send Abandoned cart reminder
...